1. Field of the Invention
This invention relates to an optical transmission system for analog signals, and in particular to a directly modulated solid-state laser. More particularly, the invention relates to the cancellation of phase modulated components arising from chirp in the semiconductor laser, which has its output distorted from its directly modulated input due to inherent nonlinearity in the laser.
2. Description of the Related Art
Directly modulating the analog intensity of a light-emitting diode (LED) or semiconductor laser with an electrical signal is considered among the simplest methods known in the art for transmitting analog signals, such as voice and video signals, on optical fibers. Although such analog transmission techniques have the advantage of substantially smaller bandwidth requirements than digital transmission, such as digital pulse code modulation, or analog or pulse frequency modulation, the use of amplitude modulation may suffer from noise and nonlinearity of the optical source.
For that reason, direct modulation techniques have been used in connection with 1310 nm lasers where the application is to short transmission links that employ fiber optic links with zero dispersion. For applications in metro and long haul fiber transmission links, the group velocity dispersion of the link requires that externally modulated 1550 nm lasers be used, but such external modulation techniques are complex and expensive. The present invention is therefore addressed to the problem of providing a simple and low cost system for direct modulation of a laser at 1550 nm so that the analog optical output can be used in metro and long haul optical networks using analog transmission while avoiding the detrimental effects of dispersion.
Direct current modulation of lasers is known for use in digital optical transmission systems such as dense wavelength division multiplexing (DWDM) systems. See, for example, Kartalopoulos, DWDM Networks, Devices, and Technology (IEEE Press, 2003), p. 154.
One of the difficulties in designing an analog system at 1550 nm is that suitable low chirp linear lasers for use at 1550 nm are not known in the prior art. One type of low chirp laser is the external cavity laser, which is used in digital optical transmission systems, and is a commercially available product.
In addition to the low chirp characteristics required for an analog optical transmission system at 1550 nm, the system must be highly linear. Distortion inherent in certain analog transmitters prevents a linear electrical modulation signal from being converted linearly to an optical signal, and instead causes the signal to become distorted. These effects are particularly detrimental to multi-channel video transmission, which requires excellent linearity to prevent channels from interfering with each other. A highly linearized analog optical system has wide application in commercial analog systems, such as broadcast TV transmission, CATV, interactive TV, and video telephone transmission.
Linearization of optical and other nonlinear transmitters has been studied for some time, but proposed solutions suffer from practical disadvantages. Most applications discussed above have bandwidths which are too large for many practical implementations. Feedforward techniques require complex system components such as optical power combiners and multiple optical sources. Quasi-optical feedforward techniques suffer from similar complexity problems and further require extremely well matched parts.
As noted above, external modulators are known for use in optical transmission systems in the prior art. U.S. Pat. No. 5,699,179 describes an externally modulated, feed-forward linearized analog optical transmitter for reducing the fiber-induced composite second order (CSO) distortion components.
Prior to the present invention, there has not been an application of an external modulator coupled to a directly (current) modulated laser for the purpose of cancelling phase modulated components arising from chirp in the external cavity laser.